Method of forming photonics structures

ABSTRACT

The disclosed embodiments relate to an integrated circuit structure and methods of forming them in which photonic devices are formed on the back end of fabricating a CMOS semiconductor structure containing electronic devices. Doped regions associated with the photonic devices are formed using microwave annealing for dopant activation.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.16/926,490, filed Jul. 10, 2020, now U.S. Pat. No. 11,402,590; which isa continuation of U.S. application Ser. No. 16/015,778, filed Jun. 22,2018, now U.S. Pat. No. 10,761,275; which is a continuation of U.S.application Ser. No. 13/600,779, filed Aug. 31, 2012, now U.S. Pat. No.10,094,988, which are incorporated herein by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

This invention was made with Government support under Agreement No.HR0011-11-9-0009, awarded by DARPA. The Government has certain rights inthis invention.

TECHNICAL FIELD

Embodiments of the invention relate generally to manufacturing ofphotonic structures and electronic devices over silicon wafers, andspecifically directed to methods of forming photonic structures at theback-end of a CMOS process flow.

BACKGROUND

Photonics over silicon have generated increasing interest over theyears, primarily for optical transmission and optical interconnects inmicroelectronics circuits. Photonic devices such as waveguides,modulators and detectors are usually formed with silicon or polysiliconand germanium materials on a semiconductor-on-insulator (SOI) or bulksilicon wafer utilizing a complementary metal-oxide semiconductor (CMOS)process. One conventional method of integrating photonic devices intothe CMOS process flow occurs at the front-end of the CMOS processingline. The typical front-end method involves first fabricating photonicdevices on a substrate and then fabricating electronic devices (e.g.,transistors) on a single CMOS wafer with different silicon materialthicknesses for the photonic devices and the electronic devices.

Front-end integration of photonic devices presents the problem that theadditional processing steps required to make the photonic devices caninterfere with the conventional CMOS process flow. For instance,front-end integration of photonic devices on a silicon on insulator(SOI) wafer requires a substrate having a thicker (>1 p.m) buried oxidematerial and a thicker (>200 nm) silicon material compared to standardCMOS electronic SOI devices which may use a substrate having a<1 μmthick buried oxide material and a<200 nm thick silicon material. Theadditional processing steps required to make photonic devices in thefront-end of the conventional CMOS processing line increases overallcomplexity and cost of an integrated circuit containing both CMOSelectronics devices and photonics devices. In addition, for aside-by-side layout of CMOS electronic devices and photonics devices,the photonic devices take up valuable substrate space that could be usedfor electronic devices. Improved methods to make photonic devices in theback-end of a CMOS processing line are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows photonic and electronic devices fabricated in a single CMOSsemiconductor structure in accordance with a disclosed embodiment;

FIG. 2 shows a method of forming photonic and electronic devices in asingle CMOS semiconductor structure in accordance with a disclosedembodiment; and,

FIG. 3 shows a method of forming photonic and electronic devices in asingle CMOS semiconductor structure in accordance with a disclosedembodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments that may be practiced. Itshould be understood that like reference numbers represent like elementsthroughout the drawings. These embodiments are described in sufficientdetail to enable those skilled in the art to make and use them, and itis to be understood that structural, material, electrical, andprocedural changes may be made to the specific embodiments disclosed,only some of which are discussed in detail below.

The terms “wafer” and “substrate” are to be understood asinterchangeable and as including silicon, silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxialmaterials of silicon supported by a base semiconductor foundation, andother semiconductor structures. Furthermore, when reference is made to a“wafer” and “substrate” in the following description, previous processsteps may have been utilized to form regions, junctions, or materiallayers in or on the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, gallium arsenide, or other knownsemiconductor materials.

Photonic devices include photonic waveguides, modulators, demodulators,and photodetectors, among other devices. Dopant activation is oftenrequired to make active photonic devices (e.g., photodetectors,modulators) function and to create ohmic electrical contact areas. Informing active photonic devices and ohmic contacts, the dopant regioncan be formed by implanting the dopant atoms into a semiconductormaterial and then activating the dopant by thermally heating it. Heatingof the dopant at a high temperature (e.g., 1000° Celsius), requires thatthe dopant activation step occur before metallization, because themetallization materials can be damaged by such high temperatures. It isfor this reason that photonics devices are formed before the completionof CMOS circuits and before formation of metallization materials whichinterconnect photonics and electrical devices. One challenge withcreating active photonic devices at the back-end of the CMOS processflow, after metallization of CMOS circuits occurs, is the use of lowtemperatures (i.e., below about 500° Celsius) to prevent damage to theCMOS circuits and metallization.

Described herein are methods of integrating photonics devices in aconventional CMOS process flow. The disclosed embodiments relate tomethods of forming photonic and electronic devices on a single CMOSsemiconductor structure at the back-end of the CMOS process flow. Thephotonic devices are formed using low deposition temperaturepolysilicon, germanium and silicon-germanium techniques such as, forexample, physical vapor deposition (PVD), chemical vapor deposition(CVD), plasma-enhanced CVD (PECVD), spin-on-glass (SOG) deposition, andatomic layer deposition (ALD). The disclosed embodiments also utilizeelectromagnetic, such as microwave, annealing at temperatures betweenabout 200° Celsius to about 500° Celsius, preferably about 300° Celsiusto about 400° Celsius, for at least five (5) minutes and up to two (2)hours for dopant activation and annealing of the active photonic devicesand ohmic contacts. Although any appropriate energy capable of beingabsorbed into the region being annealed is appropriate, for convenience,only microwave energy will be discussed hereafter. Microwave basedactivation techniques can effectively activate the desired dopants, forexample, phosphorous, antimony, gallium, boron or arsenic doping atoms,without compromising front end CMOS circuits or metallization therebyenabling the full integration of photonics devices at the back-end ofthe CMOS process flow. Low temperature microwave annealing also resultsin less dopant migration out of (and into) the dopant region in thesubstrate and results in more uniform dopant concentration within thedopant region.

In front-end integration of photonics devices in a conventional CMOSprocess, the photonic devices are usually formed with silicon orpolysilicon, germanium and silicon-germanium materials on a SOI wafer.Another advantage of the back-end integration is that photonics devicescan be formed of additional materials, which might otherwise be affectedby the processing used to form electronic devices if a front end processis used. For example, back end processing can use silicon nitride toform photonic devices, which has better photon propagation thanpolysilicon. For example, instead of fabricating a waveguide core usingsilicon or polysilicon on SOI substrates at the front end, the waveguidecore could be formed of silicon nitride on an SOI substrate in a backend process.

Referring to the drawings, where like reference numbers designate likeelements, FIG. 1 shows a partial cross-section view of one embodiment ofa semiconductor structure 200 fabricated using back end processing toform a photonics integrated structure 101 over a CMOS integratedstructure 103 on the same supporting substrate. The photonics integratedstructure 101 includes, as an example, a photodetector 250A and amodulator 250B. The CMOS integrated structure 103 includes, as anexample, an electronic device 210 fabricated as a transistor. Thesemiconductor structure 200 can be fabricated using the methodsdescribed below in connection with FIGS. 2 and 3 .

The CMOS integrated structure 103 comprises a silicon substrate 201, aburied oxide (BOX) 202 formed of, for example, silicon dioxide, asilicon fabrication material 203, a gate oxide material 219, andalternating metal and insulating materials forming an interlayerdielectric (ILD) metallization structure which includes insulation(e.g., SiO2 or BPSG) material 205, Metal 1 material 214, insulation(e.g., SiO2 or BPSG) material 206, Metal 2 material 215, last Metalmaterial 216, and a passivation material 218, for example, silicondioxide. The Metal 1 material 214 is connected by via conductors 217 tothe underlying circuitry of the electronic device 210.

The electronic device 210 is formed using the conventional CMOS processas known to those skilled in the art. The electronic device 210comprises a doped well 204, drain 211A and source 211B implant regions,a gate 212 over the gate oxide material 219 and gate sidewall spacers213. The gate 212 may be formed of polysilicon. The insulation material205 covers the electronic device 210 and the silicon fabricationmaterial 203, which is supported by the buried oxide (BOX) 202 and thesilicon substrate 201.

The photonics integrated structure 101 is formed, in this embodiment,over the CMOS integrated structure 103 and comprises a semiconductormaterial 251 formed over passivation layer 218, an oxide material 252, asilicon fabrication material 253 in which photodetector 250A andmodulator 250B are formed. Alternating metal and insulating materialsform an ILD metallization structure which includes insulation (e.g.,SiO2 or BPSG) material 255, Metal 1 material 264, insulation (e.g., SiO2or BPSG) material 256, Metal 2 material 265, last Metal material 266,and passivation material 267.

The photodetector 250A can comprise a doped or un-doped germanium (Ge)or silicon-germanium (SiGe) region 262 formed over a silicon waveguidecore 253 a. The waveguide core 253 a is surrounded by cladding materialformed by oxide material 252 and isolation regions 254 which can beformed of silicon dioxide (SiO2). The insulation material 255 alsofunctions as part of the cladding for waveguide core 253 a. Themodulator 250B can be formed as a doped or undoped silicon waveguidecore 253 b which has additionally doped regions 261A and 261B which canbe connected by conductors 257 to modulate light within waveguide core253 b. The photonics structure 101 may also contain ohmic contactregions 263A, 263B, 263C, and 263D to create ohmic contacts withconductors 257. The ohmic contact regions may be, for example, highlydoped contact regions or low temperature formed silicides, such as Nisilicides. For example, photodetector 250A can contain ohmic contactregions 263A and 263B and modulator 250B may contain ohmic contactregions 263C and 263D. Ohmic contact regions 263C and 263D may containdopant quantities higher than that of doped regions 261A and 261B. Thegermanium (Ge) or silicon-germanium (SiGe) region 262 can be used as thephoton detector in photodetector device 250A. The oxide material 252 andadditional insulation 255 and isolation regions 254 materials may beused as cladding materials surrounding the silicon waveguide cores 253 aand 253 b. The insulation material 255, which may be silicon dioxide orBPSG, covers the photonic devices 250 and the silicon fabricationmaterial 253. The photonic devices 250 may use a thicker (>1 um) oxidematerial 252 and a thicker (>200 nm) silicon fabrication material 253compared to the buried oxide (BOX) material 202 (<1 μm) and the siliconmaterial 203 (<200 nm) on which the CMOS electronic devices 210 areformed.

Alternating metal and insulating materials in the photonics structure101 form an ILD metallization structure which includes insulation (e.g.,SiO2 or BPSG) materials 255, 256, Metal 1 material 264, Metal 2 material265, last Metal material 266 and a passivation material 267. Theinsulation materials 255, 256 provide electrical and optical isolationof the photodetector 250A and modulator 250B. The Metal 1 material 264is connected by conductors 257 to the underlying photonics devices.Contact 207 connects the Metal 1 material 264 of the integrated photonicstructure 101 to the last Metal material 216 of integrated CMOSstructure 103 as an example of the electrical connection betweenstructures 101 and 103. It shall be appreciated that the semiconductorstructure 200 can be fabricated with any number of electronic andphotonic devices and with any number of contacts 207 between structures101 and 103 to form a desired electronic and photonic arrangement withinsemiconductor structure 200.

FIG. 1 is merely representative of a photonics circuit which includeswaveguides 253 a, 253 b, and associated photodetector 250A and modulator250B. However, any photonics devices can be integrated over anintegrated CMOS structure 103 using the described fabrication techniquesusing microwave activation energy for activating dopants in atemperature range of about 200° Celsius to about 500° Celsius,preferably in the range of about 300° Celsius to about 400° Celsius,which does not affect the underlying integrated CMOS circuit 103.

FIG. 2 shows a method of forming the semiconductor structure 200 usingback-end integration of the photonics devices in a CMOS process flow inaccordance with a disclosed embodiment. A CMOS semiconductor structure103 having one or more electronic devices 210 is first fabricated atstep 300 using known CMOS process techniques. The CMOS structureincludes the passivation layer 218. At step 310, associated materialsfor the photonic structure 101 are deposited over the CMOS integratedstructure 103. These include semiconductor material 251, e.g. silicon,oxide material 252, and a fabrication semiconductor material 253. Thesemiconductor material 251 is deposited over the passivation protectionmaterial 218, an oxide material 252 with appropriate thickness forforming the photonic devices 250 (e.g., >1 Rm) is deposited over thesemiconductor material 251, and a silicon fabrication material 253 withappropriate thickness for the photonic devices 250 (e.g., >200 nm) isdeposited over the oxide material 252. The materials 251, 252 and 253are deposited with low temperature deposition techniques, for example,physical vapor deposition (PVD), chemical vapor deposition (CVD),plasma-enhanced CVD (PECVD), spinon-glass (SOG) deposition, and atomiclayer deposition (ALD). At step 320, isolation regions 254, for example,trench isolation regions, are defined and etched in the siliconfabrication material 253 using, for example, photolithography, etching,fill, and chemical-mechanical polishing (CMP), to produce the isolationregions 254 at desired locations in the semiconductor fabricationmaterial 253. The isolation regions 254 define areas in the fabricationmaterial 253 where photonics devices, e.g. photodetector 250A andmodulator 250B are to be formed.

At step 330, doped regions 261A, 261B are formed within the silicon core253 b between two isolation regions 254 for modulator 250B. At this stepother doped regions may also be formed, for example, silicon waveguidecore 253 b may be doped and ohmic contact regions 263C, 263D can also bedoped. The dopants are those typically used in forming integratedcircuits, e.g. boron, phosphorus, antimony, gallium and arsenic. Thedoped regions may be formed, for example, to an atomic concentration ofabout 1×1016 to about 1×1021 dopants per cm3. In addition to theformation of doped regions, low temperature silicide materials, such asNi, may also be applied to form ohmic contact regions. At step 340, thedoped regions 261A and 261B, as well as any other doped regions andsilicide materials are annealed and activated using low temperaturemicrowave annealing. In forming active photonic devices, the dopantregion (e.g., 261A and 261B) is formed by implanting the dopant atomsinto the semiconductor material (e.g., silicon fabrication material 253)and then activating the dopant by heating it. Dopant activation of theactive photonic devices can be achieved by microwave annealing inside,for example, a cavity applicator microwave system operating at about2.45 GHz at about 1300 W, or for example by a microwave system operatingin a range of wavelengths between about 1.5 to about 8.5 GHz, althoughany suitable frequency and power can be used. Using a low temperature toactivate the dopants will not disturb the underlying CMOS structure 103.The microwave system heats the fabricated CMOS structure 103 and thepartially completed photonics integrated structure 101 to temperaturesbetween about 200° Celsius to about 500° Celsius, preferably about 300°Celsius to about 400° Celsius, for at least about 5 minutes and up toabout two (2) hours. Microwave based activation techniques caneffectively activate the desired dopants, for example, phosphorous,antimony, gallium, boron or arsenic doping atoms. Steps 330 and 340 maybe repeated to define and activate additional doping regions and ohmiccontact regions.

At step 350, the germanium (or silicon-germanium) material 262 isdeposited on the silicon material 253 a, the later of which functions asa waveguide core. Ohmic contacts 263A, 263B are also implanted into orapplied to the material 262. At step 360, an interlayer dielectricstructure (ILD) is formed using back-end insulators 255, 256 and metalmaterials (e.g., one or more of Metal 1 material 264, via 1 material256, Metal 2 material 265 and last Metal material 266) are deposited toprovide electrical contact between associated materials of the photonicsemiconductor structure and to the photonic devices 250. Also, contacts207 between structures 101 and 103 are formed. The insulator 255,together with isolation regions 254 and oxide material 252 providecladding around waveguide core 253 a. The photodetector material 262detects light within waveguide core 253 a. After all metal and insulatorlayers of the ILD are formed, at step 370, the passivation material 267is deposited on the CMOS semiconductor structure 200.

In this embodiment, dopant activation and annealing using the microwaveannealing process in step 340 occurs after the semiconductor and oxidematerials 251, 252, 253 step 310, isolation region 254 step 320 anddoped region step 330, but before the germanium (or silicon-germanium)deposition step 360. In another embodiment, as shown in FIG. 3 , thedopant activation and annealing step using low temperature microwaveannealing occurs after the germanium (or silicon-germanium) depositionstep 430 and after ohmic contact regions 263A, 263B, 263C, 263D aredoped or applied. In FIG. 3 , steps 300 to 320 are identical to the samenumbered steps in FIG. 2 . At step 430, the germanium (orsilicon-germanium) material 262 is deposited on the silicon fabricationmaterial 253, i.e. deposited on the waveguide 253 a. At step 440, inaddition to forming doped regions 261A, 261B within the silicon core 253b for modulator 250B in addition to ohmic contact regions 263C and 263D,as described above with reference to FIG. 2 , ohmic contact regions 263Aand 263B are also formed within germanium material 262 to create ohmicelectrical contact regions. Ohmic contact regions 263A and 263B may be,for example, highly doped regions or low temperature formed silicides,such as Ni silicides. Silicon core 253 b may also be doped. The dopantsare those typically used in forming integrated circuits, e.g. boron,phosphorus, antimony, gallium and arsenic. At step 450, the silicon core253 b, doped regions 261A and 261B, and ohmic contact regions 263A,263B, 263C, and 263D are activated using low temperature microwaveannealing as described above in connection to step 340 of FIG. 2 . Steps370 and 380 in FIG. 3 are identical to the same numbered steps in FIG. 2.

While disclosed embodiments have been described in detail, it should bereadily understood that the invention is not limited to the disclosedembodiments. Rather the disclosed embodiments can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described. For example, whileFIG. 1 shows a partial cross-section view of a semiconductor structure200 fabricated with an exemplary photodetector, waveguide and modulatorand an exemplary transistor, it shall be appreciated that the disclosedembodiments can be modified to fabricate the semiconductor structure 200with other photonic devices such as, for example, modulators,demodulators, light sources, as well and other electronic devices suchas transistors, diodes, and others. Accordingly, the invention is notlimited by the disclosed embodiments but is only limited by the scope ofthe appended claims.

I claim:
 1. A semiconductor structure, comprising: at least one CMOSstructure containing an electronic device formed from a firstsemiconductor material over a substrate; at least one photonic deviceformed from a second semiconductor material disposed above theelectronic device, the at least one photonic device having associatedmicrowave activated dopant implants; a first interlayer dielectric (ILD)metallization structure between the first semiconductor material and thesecond semiconductor material, configured to electrically couple theelectronic device and the at least one photonic device; and a second ILDmetallization structure disposed above the at least one photonic deviceand electrically coupled to the first ILD metallization structure by acontact passing through the second semiconductor material.
 2. Thesemiconductor structure according to claim 1, wherein the secondsemiconductor material forms part of a silicon on insulator substrateformed over the first semiconductor material.
 3. The semiconductorstructure according to claim 1, wherein the at least one photonic deviceis selected from a group consisting of a waveguide, a modulator, ademodulator and a photodetector.
 4. The semiconductor structureaccording to claim 1, wherein the second semiconductor material isthicker than the first semiconductor material from which the electronicdevice of the at least one CMOS structure is formed.
 5. Thesemiconductor structure according to claim 1, further comprising aburied oxide material with a thickness greater than or equal to 1micrometer disposed between the first semiconductor material and thesecond semiconductor material, and configured to function as a claddingfor the at least one photonic device.
 6. The semiconductor structureaccording to claim 1, wherein the at least one photonic device comprisesa waveguide including the second semiconductor material and aphotodetector material in association with the waveguide.
 7. Thesemiconductor structure according to claim 6, wherein the photodetectormaterial comprises one of germanium and silicon-germanium.
 8. Thesemiconductor structure according to claim 1, wherein the at least onephotonic device is surrounded by trench isolation regions formed in thesecond semiconductor material.